1. Field of the Invention
This invention relates to semiconductor circuits and more particularly to a charge-coupled device (CCD) circuit for compressing the dynamic range of a signal such as from a photodetector.
2. Description of the Prior Art
Many applications require compression of a signal having a large dynamic range into a signal with a smaller dynamic range to facilitate processing of the signal. For example, visible and infrared sensors or photodetectors experience a wide dynamic range. The large dynamic range sometimes exceeds the capability of charge-coupled devices which process the electrical signals obtained from the optical sensor. For example, semiconductor focal plane arrays and line arrays all fabricated on one integrated circuit chip may have their signals transferred within the chip to a suitable output terminal by utilizing a charge-coupled device chip register. The optical sensor may produce an electrical signal with the dynamic range being larger than what the charge-coupled device processor can handle.
The transformation from a large dynamic range to a smaller dynamic range is preferably a non-linear function. At low signal levels, the signal-to-noise ratio is small; thus compression of a low level signal will also compress the shot noise level associated with the signal. Attenuation of the shot noise associated with the optical signal will make the processor noise or system noise, arising from the electrical circuits, much more significant and thereby cause a reduction in the signal to noise ratio. At high signal levels, the shot noise associated with the signal is larger, hence compression of the signal will still provide a sufficient noise level to dominate over the circuit or system noise. The photon shot noise occurs because of the quantized nature of optical signals.
It is therefore desirable to provide a device for compressing the dynamic range of an input signal such that at low signal levels the attenuation is small and at large signal levels the attenuation is large, i.e. non-linear.
It is further desirable to provide a charge-coupled device for utilizing a depleted semiconductor region having therein a potential gradient towards each of at least two electrodes for dividing the charge passing into the region.
It is further desirable to provide a charge-coupled device for utilizing a depleted semiconductor region having therein a potential gradient towards each of at least two potential wells for dividing the charge passing into the region.
It is further desirable to provide a charge-coupled device for utilizing a depleted semiconductor region having therein a potential gradient towards a potential well and towards an electrode for dividing the charge passing into the region.
It is further desirable to incorporate dynamic range compression as a function of the charge in a well of a charge-coupled device by dividing any additional charge entering the region between the charge-coupled device well and either an electrode or second potential well according to the charge collected in the CCD's potential well.
It is further desirable to perform time delay integration incorporating a plurality of input stages coupled in parallel to a shift register where the charge inserted into the shift register is compressed as a function of the charge already in each particular bit position of the shift register.
It is further desirable to provide an externally programmable injection efficiency or gain adjustment for transferring charge into a charge-coupled device by dividing the input charge between the charge-coupled device and a second potential well or electrode.
It is further desirable to provide an externally programmable injection efficiency or gain which may be adjusted as a function of time to compensate for attenuation in signals such as a radar or optical signal reflected from a target at various ranges.